The context indicates a sustained commitment and determination despite setbacks.
- "Resolute" signifies firm resolve. \(\Rightarrow\) It's an ideal fit.
- "Obstinate" implies stubbornness, often negatively.
- "Relentless" describes persistence, but is generally more severe and less suitable.
- "Unwavering" is similar, but "resolute" is the most appropriate single word.
Therefore, the correct choice is resolute.
\boxed{\text{Answer: Resolute}}